Method and system for combining symmetric DSL signals and voice signals

ABSTRACT

This invention provides a simplified communication system that transmits voice and SDSL signals between customer premise equipment (“CPE”) on a single twisted-pair line. The system preferably includes at least one combiner for combining voice and SDSL signals into a combined voice/SDSL signal that can be transmitted to the CPE on a single twisted pair of wires, preferably a pair of wires currently being used to transmit the voice signal to the CPE. One combiner is ideally located at a common wiring closet of the building where the CPE is located. The combiner is ideally coupled to the existing pair of wires for the voice signal. This eliminates the need to add another set of wires between the common wiring closet and the CPE for the SDSL signal. A second combiner, normally located at or near the CPE separates the combined voice/SDSL signal into the separate voice and SDSL signals for processing by the CPE. The second and first combiners normally also combine and separate, respectively, the voice and SDSL signals transmitted from the CPE.

This application is a continuation of, and claims the benefit of, U.S.patent application Ser. No. 09/425,472, filed Oct. 22, 1999 and issuedas U.S. Pat. No. 6,693,916.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to generic Digital Subscriber Line systems andvoice communications systems and more particularly to methods andsystems for combining symmetric Digital Subscriber Line signals andanalog voice signals.

2. Description of Related Art

With the increased usage of Internet services and the desire or need forother high data rate signals, consumers require equipment that canhandle data signals at a rate greater than that available viatraditional analog voice modems, which currently have a maximum datarate of 56K bits per second (“BPS”). Recently, business and residentialconsumers are using Digital Subscriber Line (“DSL”) modems as part of asystem for getting high speed access to the Internet. DSL type ofservice is currently being offered by many telecommunication accessservice providers, which include numerous independent access providersand traditional telephone companies. There are many variations of DSLincluding High-Bit-Rate Digital Subscriber Line (“HDSL”), Asymmetric DSL(“ADSL”), and Symmetric DSL (“SDSL”). (“Asymmetric” refers to the factthat the upload and download bit rates are not equal and “Symmetric”refers to the fact that the upload and download bit rates are the same.)

HDSL and SDSL are both symmetrical DSL signals that use the samemodulation technique. HDSL systems use two pairs of twisted wire,operate at a fixed data rate (equivalent rate to T1 in North America andE1 outside North America) and are mainly used as a replacement for T1and E1 network connections. ADSL and SDSL systems use a single twistedpair of wires and are capable of operating at variable data rates. Adescription of an exemplars DSL system employing 2B1Q modulation isdescribed in Bellcore TA-NWT-001210 Issue 1, October 1991. GenericRequirements for High-Bit-Rate Digital Subscriber Lines which is herebyincorporated by reference for its teachings on DSL systems and 2B1Qmodulation.

These DSL services are generically called xDSL. xDSL service reaches itscustomers by using copper wires that extend from a telephone company'sCentral Office (“CO”) to a residence or business. Currently, telephonecompanies mostly offer ADSL service, which is a service targeted attheir consumer or residential customer base and offer businesses T1service using HDSL. Independent or alternative access providers are alsoin the access market and offer mostly SDSL. There are several types ofSDSL and HDSL modems available. The most common type of modulationformat the modems employ is a 2B1Q (two bits per quadrant or symbol)format, which is a simple four level signal modulation format known asPulse Amplitude Modulation (“PAM”). For the sake of clarity, during theremainder of the document, only SDSL will be described although theinvention applies equally to both SDSL systems and single wire pair HDSLsystems that use 2B1Q modulation.

FIG. 1 (Prior Art) is a diagram of a prior art communication system 270in which both voice and data (via SDSL access) are delivered throughindependent means and equipment to an individual customer. Voice andSDSL is delivered in this way because of the inability of voice and suchdata signals to coexist on the same connection. As shown in this figure,Customer premise equipment (“CPE”) 240 and Central Office (“CO”)equipment 250 receive (a) traditional voice communication via a twistedpair of wires 173 and (b)SDSL communication via a separate twisted pairof wires 214 commonly dedicated to transferring only SDSL signals. TheCPE 240 includes call signal destination equipment 144, an SDSL modem212, and digital signal destination equipment 148. The CO equipment 250includes a telephone switch 174 and an SDSL Modem and DSL AccessMultiplexer (“DSLAM”) 213. As shown in FIG. 1, the SDSL Modem/DSLAM 213is coupled to the Internet 180 by a Router/Internet Service Provider(“ISP”) 260, the connection thereto is known to one of skill in the art.

In this FIGURE, the central office (“CO”) equipment 250 of a telephonecompany provides voice communication on the twisted pair of wires 173between the call signal destination equipment 144 in the CPE 240 and apublic switch telephone network (“PSTN”) 190. The call signaldestination equipment 144 may be any voice signal processing apparatusor telephone system such as a telephone keyset, public branch exchange(“PBX”), or voice band modem or fax. It is noted that the PSTN 190 maybe any telephone system including PSTN, PBX, or plain old telephonesystem (“POTS”). As noted above, the CO equipment 250 includes atelephone switch 174 that transfers or switches analog voice signals tothe twisted pair of wires 173 when the signal is addressed to the CPE240 and to a connection into the public switched telephone network(“PSTN”) 190. Voice and signaling signals appear on wires 173 andconsist of voice band signals whose frequency ranges from zero to 3.4kHz, a batter feed voltage (DC at −48 volts) and an AC ringing highvoltage (20 Hz at 90 volts RMS). Transient signals may also appear onthe twisted pair of wires 173 during changes in line conditions such asat the beginning and end of a ringing signal, the taking of a phoneoff-hook during ringing and not during ringing, the placement of a phoneon-hook, the switching of ringing to battery feed signal and theswitching of the batter feed signal to the ringing signal.

As also shown in this FIGURE an SDSL access provider (either thetelephone company or an alternative service provider) uses additionalequipment, an SDSL Modem//DSLAM 213 inside the CO 260 for providing datacommunication on the twisted pair of wires 214 between the CPE 240 andthe Internet 180 via a Router/ISP 260. The SDSL Modem/DSLAM 213 includesa digital subscriber line access multiplexer (“DSLAM”) with SDSL modems.

The SDSL (2B1Q modulation format) modem inside the SDSL Modem/DSLAM 213converts SDSL signals (which are modulated analog data signals) on thetwisted pair of wires 214 to digital signals. The SDSL Modem/DSLAM 213also multiplexes many SDSL modem signals into a single high-speed signaland passes that signal on to the ISP/Router 260. The ISP/router 260interconnects the SDSL Modem/DSLAM 213 to the Internet 180.

Similarly, when a digital data signal is transmitted from the Internet180 to the CPE 240 via line 214, the ISP/router 260 transfers theInternet traffic to the SDSL Modern/DSLAM 213. Any digital data that isaddressed to the CPE 240 is then sent to the appropriate SDSL modem ofthe SDSL Modem/DSLAM 213. The SDSL modem in the SDSL Modern/DSLAM 213converts this digital data to an SDSL signal where the SDSL signal ismodulated analog signal. The SDSL signal is transmitted to the CPE 240Via the twisted pair of wires 214. The SDSL (2B1Q) signals have afrequency range starting from DC (0 Hz) to an upper frequency that isone half of the assigned data rate. For example: when the data rate ofthe service being offered is 384 kbps, the frequencies may range from 0Hz to 192 kHz.

As noted above the CPE 240 includes an SDSL modem 212 and digital signaldestination equipment 148. The SDSL modem 212 demodulates SDSL signals(as noted, the SDSL signals are analog data signals) received on thetwisted pair of wires 214 and provides a digital signal that istransferred to the digital signal destination equipment 148 via line145. Line 145 may be an Ethernet cable or serial cable capable oftransferring a digital signal. The digital signal destination equipment148 may be networking equipment such as a hub, router or switch or anindividual computer. The SDSL modem 212 also modulates digital signalsreceived from the digital signal destination equipment 148, using amodulation or encoding technique compatible with that used by the SDSLmodem of SDSL Modem/DSLAM 213, and transmits them to the SDSLModem/DSLAM 213 on the twisted pair of wires 214.

The twisted pairs of wires 173 and 214 are typically owned by the localtelephone company and stretch between the CO and a demarcation pointjust inside a building, which is known as the Minimum Point of Entry(“MPOE”). The wiring (173 and 214) that connects between the MPOE andthe CPE 240 is privately owned. In an office buildings that containsmore than one tenant, the MPOE and the privately owned wiring is housedin a single wiring closet that is common for all tenants. Thus, when theCPE 240 is located in an office of a large building, the common wiringcloset of the building may be located several hundreds of feet from thecall signal destination equipment 144 and the SDSL modem 212. In manycases when the line 214 is being installed from the CO equipment 250, aconsumer will not have the second twisted pair of wires 214 from theMPOE (or from the common wiring closet) to the CPE 240 and so will berequired to install them.

In older buildings, adding additional sets of wires for each digitalsignal destination may be prohibitively costly and also time consuming,delaying the implementation of such high speed data access. SDSLcommunication systems, however, were not designed to operate with thetelephone voice systems. SDSL signals and voice signals are incompatibleand thus must use the separate wires 173 and 214 to transfer SDSL andvoice signals from a CO to a building. One element of theirincompatibility is that SDSL signals and voice signals overlap orinterfere with each other (voice: 0 to 3.4 kHz and SDSL: 0 to hundredsof kHz). SDSL modems can not tolerate the voice signal mixed with itsmodem signal and humans can hear extremely low audio signal levels andthus also can not tolerate the SDSL signal. Another element is that SDSLmodems do not contain algorithms for handling transient line conditionssuch as those outlined above. SDSL signals are modulated using a verysimple modulation method and thus intolerant of interference and SDSLmodems do not have any external filtering.

In contrast, ADSL communication systems and equipment were designed toshare one set of twisted copper pair of wires from the CO to a buildingwith an analog voice signal from a telephone company. The systemcharacteristics of ADSL communication that allows it to coexist withvoice signals include: 1) a large non-overlap or separation of signalsbetween the maximum voice frequency (0 to 3.4 kHz) and the lowest ADSLfrequency (22 kHz and above): 2) strong analog ADSL separation filters:3) a complex ADSL modulation scheme: and 4) Digital Signal Processing(“DSP”) algorithms that operate inside the ADSL modem to handletransient line conditions.

ADSL service has several disadvantages, however, for business use overSDSL service. In particular, ADSL signals are non-symmetrical: in otherwords the download signal rate is greater than the upload signal rate(from the CPE to ADSL service provider). The asymmetry is a disadvantagefor commercial use because business customers typically need to movelarge blocks of data both to and from their establishment.

Consumers desiring the many advantages of SDSL service ideally wouldlike to make use of their existing voice-signal wiring rather thanhaving to install an additional line (part of line 214 of FIG. 1)between their CPE and a remote location in the same building (such asthe MPOE or a wiring closet). Consequently, the need exists for a systemand method that can combine SDSL signals with analog voice signals on asingle twisted pair of wires without corrupting either signal even inthe presence of voice and voice signaling signals.

SUMMARY OF THE INVENTION

The present invention includes a system for communicating a telephonyvoice signal and a SDSL signal on a single combined line where thefrequency spectrum of the telephony voice signal during steady stateconditions and the frequency spectrum of the SDSL signal may overlap.Further, the telephony voice signal may include a plurality oftransients during non-steady state conditions, including transients thatoccur when a telephony device goes off-hook while a ring signal ispresent in the telephony voice signal. The SDSL signal ideally has afour level modulation format such as 2B1Q.

The system includes a first voice and SDSL combiner and a second voiceand SDSL combiner. In the preferred embodiment, the second combiner isideally identical to the first combiner. Each combiner includes meansfor communicating the telephony voice signal and SDSL signal on the samesingle line without significantly corrupting or interrupting the SDSLsignal and telephony voice signal even when transients are present onthe telephony voice signal.

The first combiner is coupled to a telephone switch on a first voiceline for communicating the telephony voice signal, to a first SDSLdevice on a first data line for communicating the SDSL signal, and tothe single combined line for communicating a signal that is acombination of the telephony voice signal and the SDSL signal.

The second combiner is coupled to a telephony device on a second voiceline for communicating the telephony voice signal, to a second SDSLdevice on a second data line for communicating the SDSL signal, and tothe first combiner on the single combined line for communicating thesignal that is a combination of the telephony voice signal and the SDSLsignal.

In one embodiment the first (and also the second) combiner includes twosignal processing means: a) a voice signal processing means that iscoupled to a voice line and to the single combined line and b) an SDSLsignal processing means that is coupled to a data line and to the singlecombined line.

The voice signal processing means attenuates the frequency spectrum ofthe telephony voice signal from the voice line above a firstpredetermined cutoff frequency and attenuates the frequency spectrum ofthe signal sent to the voice line from the single combined line abovethe first predetermined cutoff frequency. These attenuations are ideallyaccomplished by means of a single low pass filter having a cutofffrequency higher than the upper frequency spectrum of the telephonyvoice signal during steads state conditions. The low pass filter alsosuppresses the effect of undesirable transients by means of a pluralityof diodes coupled in parallel with each inductor of a plurality ofinductors coupled closest to the voice line.

The SDSL signal processing means attenuates the frequency spectrum ofthe SDSL signal from the data line below a second predetermined cutofffrequency and attenuates the frequency spectrum of the signal sent tothe data line from the single combined line below the secondpredetermined cutoff frequency. These attenuations are ideallyaccomplished by means of a single high pass filter, which ideally is alow order filter, having a cutoff frequency higher than that of the lowpass filter in the voice signal processing means.

The present invention also includes a method for communicating atelephony voice signal and an SDSL signal on a single combined linewhere the frequency spectrum of the originating telephony voice signalduring steady state conditions and the frequency spectrum of theoriginating SDSL signal may overlap. The originating telephony voicesignal may also include a plurality of transients during non-steadystate conditions.

The method includes communicating die telephony voice signal between atelephone switch on a first voice line and a telephony device on asecond voice line via a single combined line. Also, the method includescommunicating the SDSL signal between a first SDSL processing device ona first data line and a second SDSL processing device on a second dataline via the same single combined line. The method includescommunicating on the single combined line without significantlycorrupting or interrupting the SDSL signal and the telephony voicesignal even when transients are present on the telephony voice signal.

The method may attenuate the frequency spectrum of the signals betweenthe first and second voice lines above a first predetermined cutofffrequency and attenuate the frequency spectrum of the signals betweenthe first and second data lines below a second predetermined cutofffrequency that is higher than the first predetermined cutoff frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art diagram of a communication system 270, in whichthe system provides an analog voice signal between customer premiseequipment 240 and a central office 250 and provides an SDSL signal(using an SDSL service) between the customer premise equipment 240 andthe central office 250.

FIG. 2 is a diagram of an exemplary communication system 200 inaccordance with the present invention, in which the system provides ananalog voice signal on line 173 between a central office 250 and thebuilding that contains the customer premise equipment 220 and providesan SDSL signal (using an SDSL service) on line 214 between the centraloffice 250 and the building that contains the customer premise equipment220 and in which the analog voice signal and the SDSL signal arecombined at a first voice and SDSL combiner 11 located in or near thebuilding that contains the CPE 220 therefore allowing the consumer touse only one twisted-pair line between their CPE 220 and the location ofthe combiner 11.

FIG. 3 is a block diagram of an exemplary voice and SDSL combiner 10 inaccordance with the present invention in which the device combines andseparates analog voice signals and SDSL signals as required in thesystems depicted in FIGS. 2 and 4.

FIG. 4 is a diagram of a simplified communication system 30 thatincludes a system for combining a voice signal and an SDSL signal on asingle combined line in accordance with the present invention.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, the preferred embodiment and examples shownshould be considered as exemplars, rather than as limitations on thepresent invention.

FIG. 2 is a diagram of an improved system 200 for transmitting an analogvoice signal between customer premise equipment (“CPE”) 220 and acentral office 250 of a telephone service provider, and an SDSL signalbetween the CPE 220 and an SDSL service provider equipment 213 locatedin the central office 250 according to the present invention. The analogvoice signal on line 173 and the SDSL signal on line 214 are combinedand separated by the first and second voice and SDSL combiners 11 and12, as described below.

The system 200 shown in FIG. 2 includes equipment known to those skilledin the art and which operate as already described in FIG. 1, as follows:the PSTN 190, the central office 250, the analog voice twisted pair ofwires 173, the call signal destination equipment 144, the Internet 180,the Router/ISP 260, the SDSL twisted pair of wires 214, the SDSL modem212, the digital signal destination equipment 148, and line 145 couplingthe modem 212 to the equipment 148.

The preferred embodiment 200 also includes a first voice and SDSLcombiner 11, ideally situated at a location (which may be a commonwiring closet) in or near the building that contains the CPE 220, nearthe existing voice signal wires 173 and 210, and near where the twistedpair of wires 214 supplied by the SDSL service provider terminate, suchthat additional wiring is minimized. The first voice and SDSL combiner11 is coupled to the telephone switch 174 of the CO 250 via the analogvoice line 173 and to the SDSL Modem/DSLAM 213 of the CO 250 via analogdata line 214. The first voice and SDSL combiner 11 combines the analogvoice signal on line 173 with the SDSL signal on line 214 to generate acombined voice/SDSL signal on line 210.

A second voice and SDSL combiner 12, ideally identical to the firstvoice and SDSL combiner 11 and ideally located relatively close to theother equipment of the customer as part of the CPE 220, receives thecombined voice/SDSL signal on line 210 and separates the signal into ananalog voice signal on line 143 and an SDSL signal on line 216. Thesecond voice and SDSL combiner 12, also combines the analog voice signalon line 143 and the SDSL signal on line 216 into the combined voice/SDSLsignal on line 210.

As described below, in the exemplary embodiment the analog voice signalon line 143 is substantially the same as the analog, voice signalreceived on line 173. Likewise, the SDSL signal on line 216 issubstantially the same as the SDSL signal received on line 214, exceptfor possible attenuation of a baseband segment (baseband or DC to apredetermined frequency) of the frequency spectrum of the SDSL signal.However, the SDSL modem 212 processes the signal using essentially thefull frequency spectrum, which results in acceptable or manageablesignal errors. More critically, the combiner 11 in conjunction withcombiner 12 prevents or eliminates the corruption or further attenuationof the SDSL signal even when it is combined with a voice signal duringvoice signal transients, while not including or requiring complex DSPcircuitry as in prior art systems including ADSL systems. Although theabove description explains how the voice and SDSL combiners 11 and 12process signals traveling toward the CPE 220, it should be understoodthat the combiners, being ideally identical, also process signalstraveling away from the CPE 220 in essentially the same way.

As noted in the background of the invention, one of the limitations ofprior art voice/data communication systems was that an additional pairof wires may need to be installed between some location (such as acommon wiring closet as described above) and the customer premiseequipment (CPE). By placing combiners 11 and 12 as described above, anexisting twisted pair of wires 210 that is currently being used totransmit only analog voice signals may be used to transmit both analogvoice signals and SDSL signals. This resolves limitations of the priorart systems by saving the time and cost of installing additional twistedpairs of wires for each SDSL signal being transmitted from one locationin a building to a CPE elsewhere in the building where the CPE alreadyhas a twisted pair of wires connected to the location for analog voicetransmissions.

FIG. 3 presents an exemplar, voice and SDSL combiner 10 suitable for useas the first and second voice and SDSL combiners 11 and 12. As shown inFIG. 3, the combiner 10 includes a voice signal filtering section 20, aSDSL signal filtering section 50, a voice signal two-wire line or port15, a combined voice/SDSL signal two-wire line or port 14, and a SDSLsignal two-wire line or port 16. In summary, combined analog voice/SDSLsignals that are received at port 14 are processed by the voice signalfiltering section 20 and bit the SDSL signal filtering section 50. Inaddition, voice signals that are received at port 15 and SDSL signalsthat are received at port 16 are processed by sections 20 and 50respectively to provide a combined analog voice/SDSL signal at port 14.

The voice signal filtering section 20 provides voice signals on line 15by filtering or extracting voice signals from the combined voice/SDSLsignals on line 14. The voice signal filtering section 20 also filtersvoice signals from line 15 to allow them to be combined with the SDSLsignal, and includes circuitry that prevents voice signal transientsfrom affecting or noticeably corrupting the SDSL signal.

The SDSL signal filtering section 50 provides SDSL signals on line 16 byfiltering or extracting SDSL signals from the combined voice/SDSLsignals on line 14. The SDSL signal filtering section 50 also filtersSDSL signals from line 16 to allow them to be combined with the voicesignal. The SDSL signal filtering section 50 does not impact the voicesignal or impact SDSL modems coupled to the voice and SDSL combiner 10.

In order to effectively separate voice signals and SDSL signals fromcombined voice/SDSL signals received on line 14, the combiner 10 shouldnot cause detectable distortion to either signal and also not impactdevices that are commonly connected to the combiner 10 as noted. Forexample, as shown in FIG. 2, an SDSL modem may be connected to the port16 for receiving SDSL signals generated by the combiner 10 from combinedvoice/SDSL signals received on line 14. In order to effectively separatethese signals and not impact device commonly connected to combiner 10,the possible signals on line 14 should be considered in particulartransients in the voice signal.

During steady state as noted above, the voice signal occupies afrequency band of zero to 3.4 KHz. During a call setup or completion,transients may be generated in the analog voice signal. Such transientsinclude ringing generation of pulse tones, and taking a phone off-hookduring ringing or non-ringing. The SDSL signal may have a frequencyspectrum that extends from baseband or DC to a predetermined upper limitfrequency as a function of the encoding technique used in the SDSLmodems and the data rate of the SDSL signal. In the preferredembodiment, a lower section or baseband section of the frequencyspectrum of the SDSL signal is attenuated in order to enable the voicesignal to be combined with the attenuated SDSL signal during steadystate of the voice signal. This design criterion (the attenuation of abaseband section of the SDSL signal) is made known to the SDSL serviceprovider and customer. As noted, the design criterion does not introducesignificant errors in the SDSL modem or resultant digital data.

Accordingly, given the known upper frequency of the frequency spectrumof the analog voice signal during steady state, the combiner 10 ideallyfilters the combined voice/SDSL signal to extract the voice signal usinga low pass filter having a cutoff frequency substantially higher thanthe upper voice frequency. This reduces possible distortion of the voicesignal due to the filter. Further, the combiner 10 ideally filters thecombined voice/SDSL signal to extract the SDSL signal using a high passfilter having a cutoff frequency greater than that of the low passfilter that is used for the voice signal. This selection of the cutoffsfor the low pass filter and high pass filter helps prevent spectrumleakage from affecting the voice and SDSL signals during filtering.

As noted above, a key concern or problem with combining analog voicesignals and SDSL signals is the possible corruption of the SDSL signalduring voice signal transients. An SDSL modem receiving gun SDSL signalwith such corruption may not be able to extract ant data information inthe signal because the modem does not include DSP algorithms designed tocompensate for signal corruption. As noted above, the possible analogvoice signal transients include 1) the introduction of a ring voltage inthe voice signal; 2) the DC voltage/current of the voice signal; 3) theintroduction of dial tones or pulses in the voice signal; 4) the changein DC voltage/current of the voice signal when a telephone devicecoupled to the voice line goes off-hook (shorting the line); and 5) thechange in ring voltage present on the voice signal when a telephonedevice coupled to the voice line goes off-hook (shorting tile line)during the presence of the ring voltage (during ringing). The combiner10 ideally should be able to prevent any of these signals fromdisturbing or influencing the SDSL signal in such a manner as to disruptthe SDSL modem's demodulation process (act of receiving and correctlydecoding the information within the SDSL signal) and thus preventproducing errors in the resultant digital data stream. In particular,the combiner 10 should be able to provide undistorted SDSL signals online 16 even when voice signal transients are present. The circuitry ofthe combiner 10 and the mechanics thereof that suppresses the effects ofvoice signal transients on the SDSL signal are presented below.

It is also noted that the high pass filter for the SDSL signals shouldnot add significant or greatly variable group delay to the SDSL signal.As noted above, an SDSL modem is coupled to line 16 of combiner 10.These modems include equalization circuitry that attempts to equalizedelays caused by the transmission of the SDSL signal from one SDSL modemto another. The introduction of group delay by the high pass filter mayplace an additional burden on the equalization circuitry of thesemodems. The group delay may introduce data errors when the equalizationcircuitry is not able to fully compensate for the group delay introducedby the filter. Accordingly, the high pass filter should be designed sothe filter does not introduce unequalizable group delays.

It is further noted that, because the low pass filter and high passfilter are commonly coupled to the same line, the impedance of onefilter may be projected onto the other filter. In prior art combiners(such as ADSL splitters), the impedance to high frequency signals, aspresented by the low pass filter, may distort the higher frequencysignals of the SDSL signal. The voice signal filter section 20 uses alow pass filter that overcomes the limitations of the prior art, and theSDSL signal filter section 50 uses a high pass filter that overcomes thelimitations of the prior art.

Regarding the voice signal filtering section 20, the cutoff of its lowpass filter (“LPF”) should be much higher than the upper frequencyspectrum of the voice signal during steady state. In the preferredembodiment of the invention, the cutoff is at least twice as great asthe 3.4 KHz upper range of the standard telephony voice signal, i.e., itis greater than 6.8 KHz, and ideally in the range of 7.5 KHz to 8.0 KHz.The selection of this cutoff for the LPF of the voice signal filteringsection 20 reduces the impact of the filtering section 20 on voicesignals. As shown in FIG. 3 the voice signal filtering section 20includes a plurality of capacitors 22, 24, 26, 34, and 42, twotransformers acting as inductors 28 and 32, and a plurality of diodes36, 38, 44, and 46.

When capacitors 22, 24, 26, 34, and 42 have a nominal capacitance of 22NF, 33 NF, 6.8 NF, 10 NF, and 10 NF, respectively, and transformers 28and 32 have an effective nominal inductance of 76 MH and 64 MH,respectively, the LPF has a frequency response as shown in TABLE 1,which lists the attenuation of the filter in dB as a function offrequency. Viewing TABLE 1. it can be seen that the cutoff of the LPF ofthe voice filtering section 20 is about 7.5 KHz, as desired.

TABLE 1 LOW PASS FILTER HIGH PASS FILTER RESPONSE RESPONSE Hz dB kHz dB300 −0.019 2 87.496 500 −0.014 4 63.556 700 −0.005 6 49.694 900 0.006 839.991 1100 0.020 10 32.581 1300 0.038 12 26.628 1500 0.058 14 21.6841700 0.082 16 17.490 1900 0.108 18 13.894 2100 0.138 20 10.811 23000.170 22 8.203 2500 0.205 24 6.055 2700 0.242 26 4.351 2900 0.280 283.059 3100 0.321 30 2.119 3300 0.362 32 1.458 3500 0.403 34 1.003 37000.474 36 0.694 3900 0.484 38 0.484 4100 0.523 40 0.341 4300 0.559 420.243 4500 0.593 44 0.175 4700 0.625 46 0.128 4900 0.656 48 0.094 51000.688 50 0.070 5300 0.724 52 0.052 5500 0.772 54 0.040 5700 0.839 560.030 5900 0.940 58 0.023 6100 1.094 60 0.018 6300 1.327 62 0.013 65001.669 64 0.010 6700 2.153 66 0.008 6900 2.805 68 0.006 7100 3.644 700.004 7300 4.672 72 0.003 7500 5.877 74 0.002 7700 7.237 76 0.001 79008.729 78 0.001 8100 10.334 80 0.000 8300 12.041 82 −0.001 8500 13.847 84−0.001 8700 15.762 86 −0.001 8900 17.806 88 −0.002 9100 20.021 90 −0.0029300 22.473 92 −0.003 9500 25.277 94 −0.003 9700 28.661 96 −0.003 990033.151 98 −0.003 10100 40.555 100 −0.004

As noted above, there may be many different types of signals present online 14, in particular the voice signal transients as described above,that the combiner 10 should ideally handle without causing interferenceto other signals present on the line 14. Transient signals generated bytelephony voice signals may introduce significant handling problems. Ofthe possible transients that may occur in a telephony voice signal,compensating for a telephone device going off-hook (producing a short onport 15) while a ring voltage is present on line 14 is particularlyimportant. For example, the telephone switch 22 of FIG. 4 may produce aringing signal while the analog voice device 26 goes off-hook. Thisproduces a transient in the voice signal that exists between thetelephone switch 22 and the analog voice device 26 of FIG. 4.

The two combiners 11 and 12 should be able to suppress the transients sothat an SDSL signal between the SDSL modems 20 and 24 of FIG. 4 is notaffected. During this kind of transient the transformers 28 and 32(acting as inductors) of each combiner 11 and 12 may produce a largespike of voltage up to 300 volts due to the presence of the ringingsignal. The filters present in the combiners 11 and 12 are designed toprocess steady state signals, so such a voltage spike would not besignificantly attenuated by the filters and thus would corrupt the SDSLsignal between the SDSL modems 20 and 24. Such an interruption maydisrupt the operation of the SDSL modems whereby the modems losesynchronization and where the lost of synchronization between the modemmay lead to substantial data loss.

In order to reduce or prevent the first transformer 28 of each combiner11 and 12 from producing such a spike during this transient, thepreferred embodiment 10 includes the plurality of diodes 36, 38, 44, and46. Diodes 36 and 38 are placed in parallel with one winding of thetransformer 28 and diodes 44 and 46 are placed in parallel with theother winding of the transformer 28. When the above referenced transientoccurs (when an open line occurs on line 15 while a ring voltage ispresent on line 14) the diodes 36, 38, 44, and 46 prevent thetransformer 28 from generating a spike of voltage that may feed back tolines 14 and 16. Thus, these diodes help prevent possible corruption orinterruption of SDSL signals present on lines 14 and 16 due to thistransient in the voice signal. The transformer 32 is less likely tointroduce or produce a voltage spike when such a transient occursbecause capacitors 24 and 26 buffer such a transient before it reachesthe transformer 32.

As noted above, combiner 10 ideally should also not burden theequalization process of an SDSL modem coupled to line 16. The SDSLsignal filtering section 50 includes a high pass filter (“HPF”). The HPFreduces voice signals received from line 14 so that primarily only SDSLsignals from line 14 are transmitted to line 16. Filters introduce groupdelay to signals as they pass through. The presence of a complex filtermay add variable group delay that is not easily equalized, increasingthe complexity or burden of associated modems. The HPF of the preferredSDSL signal filtering section 50 has a relatively short group delay and,in addition, the difference between the minimum group delay introducedby the HPF and the maximum group delay introduced by the HPF is small.This reduces the complexity of associated modems because the delayintroduced by the combiner 10 is relatively constant and small.

In the preferred embodiment 10, the data filtering section 50 includestwo capacitors 52 and 54 and an inductor 56. The two capacitors 52 and54 and the inductor 56 are configured as shown in FIG. 3 to form an HPFthat filters or reduces the presence of signals having a frequencyspectrum lower than the cutoff of the filter. When capacitors 52 and 54have a nominal capacitance of 33NF, and inductor 56 has a nominalinductance of 0.390MH, the HPF filter of the data filtering section 50has a frequency response as shown in TABLE 1, which lists theattenuation of the filter in dB as a function of frequency. ViewingTABLE 1, it can be seen that the cutoff of the HPF of the data filteringsection 50 is about 24KHz. It is noted that this cutoff frequency mayaffect SDSL signals on line 14 in some embodiments, in particular for aportion of the baseband spectrum below the cutoff of this filter and ofthe low pass filter as described above. This cutoff is selected,however, to reduce interference between the voice signal filteringsection 20 and the SDSL signal filtering section 50.

As noted above, the cutoffs of the LPF and HPF of sections 20 and 50 areideally separated to reduce interference between the filters. As notedabove each filter also generates signal impedance at the input of theother filter. For example, the LPF of section 20 generates signalimpedance at the input of the HPF of section 50. Thus, if the LPF ofsection 20 presents low shunt impedance to line 14 at high frequenciesit may suppress or reduce the high frequency signals present in SDSLsignals on line 14. An LPF commonly terminates with a capacitor that maypresent low shunt impedance at the input of a parallel HPF. However, asshown in FIG. 3 in the preferred embodiment 10, the LPF of the voicesignal filtering section 20 terminates with an inductor (transformer 32acting as an inductor). The transformer 32 has greater impedance as thefrequencies increase and thus is less likely to short SDSL signals online 14. Thus, it can be seen that the combiner 10 may be used toseparate combined analog voice and SDSL signals while not causingsignificant distortion of either signal, even when transients occur inthe voice signal.

It is noted that other HPF and LPF configurations may be used in thecombiner 10 where the filters have similar characteristics but notnecessarily the same cutoff frequencies. Accordingly, it is to beunderstood that the invention is not to be limited by the specificillustrated embodiment, but only by the scope of the appended claims.

1. An apparatus for communicating telephony voice signals with symmetricdigital subscriber line (SDSL) signals, comprising: a combiner includinga first port configured to receive a telephony voice signal, a secondport configured to receive an SDSL signal, a third port configured to becoupled to a single communication medium to enable the simultaneoustransmission of the voice and SDSL signals over the single communicationmedium, and circuitry configured to process the telephony voice signaland SDSL signal to attenuate at least a portion of partially interferingsignals that exist in an overlap frequency range.
 2. The apparatusaccording to claim 1, wherein the circuitry comprises a voice signalfiltering section coupled to said first port and to said third port,said voice signal filtering section being configured to attenuate thefrequency spectrum of the voice signal above a first predeterminedcutoff frequency and an SDSL signal filtering section coupled to saidsecond port and to said third port, said SDSL filtering section beingconfigured to attenuate the frequency spectrum of the SDSL signal belowa second predetermined cutoff frequency.
 3. The apparatus according toclaim 1, wherein the single communication medium is a single twistedpair of wires.
 4. The apparatus according to claim 1, wherein thefrequency spectrum of the telephony voice signal during steady stateconditions and the frequency spectrum of the SDSL signal may overlap,and wherein the telephony voice signal may include a plurality oftransients during non-steady state conditions, and wherein the voicesignal and SDSL signal filtering sections are configured to communicateboth the voice and SDSL signals, via the third port, onto the singlecommunication medium without significant interference even whentransients are present on the voice signal.
 5. The apparatus accordingto claim 2, wherein the voice signal filtering section includes a lowpass filter having a cutoff frequency higher than an upper frequencyspectrum of the telephony voice signal during steady state conditions,and the SDSL signal filtering section includes a high pass filter havinga cutoff frequency higher than the cutoff frequency of the low passfilter.
 6. The apparatus according to claim 5, wherein both thetelephony voice signal and the SDSL signal are processed to attenuateoverlapping signals.
 7. The apparatus according to claim 6, wherein thetelephony voice signal is processed with said low pass filter and theSDSL signal is processed with said high pass filter.
 8. The apparatusaccording to claim 6, wherein the low pass filter includes a pluralityof capacitors and a plurality of inductors and at least one of theinductors of the low pass filter is coupled to the single communicationmedium.
 9. The apparatus according to claim 8, wherein the low passfilter farther includes a plurality of diodes, and at least one of theplurality of inductors of the low pass filter is coupled in parallelwith the plurality of diodes.
 10. The apparatus according to claim 6,wherein the high pass filter comprises a low order filter.
 11. Theapparatus according to claim 10, wherein the high pass filter is formedof an inductor in parallel with the second port, and two capacitors inseries with the second port.
 12. An apparatus for communicatingtelephony voice signals with digital data symmetric digital subscriberline (SDSL) signals, comprising: a voice signal port configured toreceive a telephony voice signal; an SDSL signal port configured toreceive an SDSL signal; a combined voice/SDSL signal port configured tobe coupled to a two-wire communication line; a voice signal filteringsection coupled to said voice signal port and to said combinedvoice/SDSL signal port, said voice signal filtering section including alow pass filter having a cutoff frequency higher than an upper frequencyspectrum of the telephony voice signal during steady state conditionsand being configured to attenuate a frequency spectrum of the voicesignal above a first predetermined frequency; and an SDSL signalfiltering section coupled to said SDSL signal port and to said combinedvoice/SDSL signal port, said SDSL filtering section including a highpass filter having a cutoff frequency higher than the cutoff frequencyof the low pass and being configured to attenuate a frequency spectrumof the SDSL signal below a second predetermined frequency; wherein theapparatus enables simultaneous reception of telephony voice and SDSLsignals via said voice signal and SDSL signal ports, respectively, andtransmission of combined voice and SDSL signals via a two-wirecommunication line coupled to said combined voice/SDSL signal port; andwherein the apparatus further enables reception of combined voice andSDSL signals via said two-wire communication line and transmission ofvoice and SDSL signals via the voice and SDSL signal ports,respectively.
 13. The apparatus according to claim 12, wherein thefrequency spectrum of the telephony voice signal during steady stateconditions and the frequency spectrum of the SDSL signal may overlap,and wherein the telephony voice signal may include a plurality oftransients during non-steady state conditions, and wherein the voicesignal and SDSL signal filtering sections are configured to communicateboth the voice and SDSL signals, via the third port, onto the singlecommunication medium without significant interference even whentransients are present on the voice signal.
 14. The apparatus accordingto claim 12, wherein both the telephony voice signal and the SDSL signalare processed to attenuate overlapping signals.
 15. The apparatusaccording to claim 12, wherein the low pass filter includes a pluralityof capacitors and a plurality of inductors and at least one of theinductors of the low pass filter is coupled to the two-wirecommunication line.
 16. The apparatus according to claim 15, wherein thelow pass filter further includes a plurality of diodes, and at least oneof the plurality of inductors of the low pass filter is coupled inparallel with the plurality of diodes.
 17. The apparatus according toclaim 12, wherein the high pass filter comprises a low order filter. 18.The apparatus according to claim 17, wherein the high pass filter isformed of an inductor in parallel with the SDSL signal port, and twocapacitors in series with the SDSL signal port.
 19. A method forcommunicating a telephony voice signal and a symmetric digitalsubscriber line (SDSL) signal on a single combined line, where thefrequency spectrum of the telephony voice signal during steady stateconditions and the frequency spectrum of the SDSL signal may overlap,and where the telephony voice signal may include a plurality oftransients during non-steady state conditions, the method comprising:receiving a telephony voice signal; receiving an SDSL signal; processingsaid telephony voice signal with a low pass filter having a cutofffrequency higher than an upper frequency spectrum of the telephony voicesignal during steady state conditions to attenuate a frequency spectrumof the voice signal above a first predetermined frequency; processingsaid SDSL signal with a high pass filter having a cutoff frequencyhigher than the cutoff frequency of the low pass to attenuate afrequency spectrum of the SDSL signal below a second predeterminedfrequency; and transmitting combined voice and SDSL signals to a remotelocation via a two-wire communication line.
 20. A method as recited inclaim 19, further comprising receiving combined voice and SDSL signalsvia said two-wire communication line and outputting voice and SDSLsignals via separate voice and SDSL signal ports, respectively.
 21. Amethod as recited in claim 19, wherein the SDSL signal is modulatedusing a four level format.
 22. A method as recited in claim 19, whereinthe SDSL signal is modulated using a 2B1Q format.
 23. A method asrecited in claim 19, wherein the attenuation of the telephony voicesignal includes the suppression of transients that are of a highervoltage than that of the telephony voice signal during steady stateconditions.
 24. A method as recited in claim 19, wherein the method ofattenuating the telephony voice signal has a minimum effect on the SDSLsignal.
 25. A method as recited in claim 19, wherein the method ofattenuating the SDSL signal is a low order attenuation method.